1. Field of the Invention
This invention relates generally to the field of computer graphics and, more particularly, to a system and method for correcting the presentation of color by one or more display devices (e.g. projection devices).
2. Description of the Related Art
A light beam may be described as a superposition of beams having a continuum of wavelengths. The amount of power concentrated at each wavelength λ of the continuum is given by a function F(λ) known as the power spectrum. The power spectrum may be measured by a spectrum sensing device such as a spectroradiometer. The power spectrum determines the perceived color of the light beam for a given observer. The space of possible power spectra is infinite dimensional. However, because the human eye has only three types of color sensitive cells, the space of perceived colors is generally considered to be three dimensional. Thus, the mapping between the power spectra and perceived colors is many to one. In other words, an infinite collection of power spectra may induce the same color perception for a given observer. Two power spectra that give the same color perception are said to be “metamers”.
An image on a display screen (or projection screen) comprises an array of physical pixels. Each physical pixel radiates a light beam to the observer's eye(s). Each pixel light beam has a power spectrum that determines the perceived color of the corresponding physical pixel.
Display devices generate color by mixing varying amounts of Q fundamental colors, where Q is an integer representing the number of fundamental colors. Typically, Q equals three and the fundamental colors are red, green and blue. Thus, each pixel light beam may comprise a red component beam, a green component beam and a blue component beam having power spectra ƒ1(λ), ƒ2(λ) and ƒ3(λ) respectively. Therefore, the pixel power spectrum Fγ(λ) is a linear combination of the three component spectra:                     F        γ            ⁢              (        λ        )              =                  ∑                  i          =          1                3            ⁢                        γ          i                ·                              f            i                    ⁢                      (            λ            )                                ,where the scalar values γ1, γ2 and γ3 control the relative amounts of red, green and blue respectively which are combined in the pixel light beam. Let γ denote the vector whose components are the scalar values γ1, γ2 and γ3, i.e.   γ  =                    [                              γ            1                    ,                      γ            2                    ,                      γ            3                          ]            t        .  The vector γ may be referred to herein as the color intensity vector. The display device receives a video signal that determines the vector γ for each pixel in the pixel array. The video signal may be an analog or digital video signal. The red, green and blue beams comprising the pixel beam are referred to herein as color component beams, and their corresponding spectra are referred to herein as color component spectra.
For various reasons, the color component spectra ƒ1(λ), ƒ2(λ) and ƒ3(λ) of a pixel beam may change with the passage of time. Thus, the perceived color C(t) of the pixel beam may vary in time even though the color intensity vector γ is held constant. For example, in certain types of projection devices, the color component beams are generated by passing beams of white light through red, green and blue color filters respectively. The color filters experience thermal stresses due to the absorption of light energy. The thermal stresses may, over time, induce changes in the absorption properties of the color filters. Also, the absorbing materials in the color filters may change their filtering characteristics over time as they age. Thus, there exists a need for a system and method which can correct and stabilize the color generated by displayed pixels in spite of time variation in their color component output spectra.
Suppose the pixel array is parameterized by a horizontal pixel index I and a vertical pixel index J. In addition to variations with respect to time, the color component output spectra ƒ1(λ), ƒ2(λ) and ƒ3(λ) may vary spatially, i.e. with respect to indices I and J. For example, a color filter used in the projector may have non-uniform absorption properties across its surface. Thus, the perceived color C(I,J) observed on a display (or projection screen) may vary spatially even when all pixels of the pixel array are driven by the same color intensity vector γ. Thus, there exists a need for a system and method which can correct and uniformize the spatial distribution of color generated by a display device (e.g. a projection device) to compensate for these spatial variations.
Suppose multiple units of a given model of display device (e.g. projection device) are generated in a manufacturing batch. Because of the difficulty of exactly reproducing all manufacturing conditions from unit to unit, the color component output spectra ƒ1(λ), ƒ2(λ) and ƒ3 (λ) generated by pixels of a first unit may not agree with the corresponding spectra of a second unit. Thus, the color C(1) generated by the first unit may not agree with the color C(2) generated by the second unit even when both units are driven by the same color intensity vector γ. In particular, the color C(I,J,1) generated by pixel (I,J) in the first unit may not agree with the color C(I,J,2) generated by the corresponding pixel (I,J) in the second unit, even when both units are driven by the same color vector γ. This problem may be especially noticeable when the multiple units are used in a single display system. For example, multiple units of a given model of projector may be used to project an integrated image onto a common projection screen. The non-repeatability of color between the units may be especially offensive in areas of the projection screen where projected images overlap. Thus, there exists a need for a system and method which could correct and uniformize the presentation of color between multiple units of a given display device, especially where the multiple units are employed in a single display system.
The problem of non-repeatable color is exacerbated when a display system uses display devices (a) from different manufacturers, (b) from the same manufacturer but conforming to different models, and/or (c) based on differing technologies. For example, a first display device may use LCD technology while a second display device uses digital micro-mirror technology (e.g., Texas Instruments DLP™ technology). In another example, a first projection device and second projection device may use different light sources and/or color filter materials. Thus, the color C(A) generated by a first display device A may not agree with the color C(B) generated by a second display device B even when the two display devices are driven with the same color intensity vector γ. Therefore, there exists a need for a system and method which can correct and uniformize the presentation of color between multiple display devices from potentially different manufacturers and/or based on different underlying technologies, etc.